This invention relates to a process for the production of high-purity polycarbonates and to a process for the purification of solutions which contain polycarbonate.
In order to produce polycarbonates by what is termed the phase boundary method, dihydroxydiarylalkanes in the form of their alkali salts are reacted with phosgene in heterogeneous phase in the presence of inorganic bases such as caustic soda and in the presence of an organic solvent in which the polycarbonate product is readily soluble. During the reaction, the aqueous phase is distributed within the organic phase, and after the reaction the organic phase which contains the polycarbonate is separated from the aqueous phase and is repeatedly washed with an aqueous liquid in order to remove electrolytes, residual monomers and catalyst, amongst other contaminants, and the washing liquid is subsequently separated as extensively as possible.
After washing the organic solution which contains polycarbonate with an aqueous washing liquid, a milky, turbid liquid is obtained. Separation of the aqueous phase, which contains salts, from the organic phase which contains polycarbonate, is usually effected in a plurality of stages by means of what are termed phase separation devices which operate in the manner of centrifuges and in which the organic solution is separated from the aqueous liquid due to the different densities thereof.
A purification process for polycarbonates is known from DE 195 10 061 A1 in which the mixture which is obtainable from the production process, and which comprises an organic phase which contains polycarbonate and an aqueous phase which contains salts, is separated into these two phases in a first separating stage, the organic phase is subsequently washed with a dilute mineral acid in order to remove any catalyst which possibly remains, the resulting two-phase mixture is next separated from the bulk of the aqueous phase in a phase separation device which operates in the manner of a centrifuge, the remaining organic phase which contains polycarbonate is optionally passed through one or more coalescers to remove adhering residual water which contains salts, and the washing procedure, as well as the separation of the phases, are repeatedly carried out again. The coalescers which are used in this process are packed with a material with a strongly hydrophobic surface, such as polypropylene fibres or polytetrafluorethylene fibres.
Compared with this known process, the underlying object of the present invention is to provide a purification process for the purification of solutions which contain polycarbonate, and is to provide a process for the production of polycarbonates which is improved as regards its susceptibility to malfunctions and as regards its economics and which in addition enables an even purer final product to be obtained, i.e. a purer polycarbonate, copolycarbonate or polyester carbonate.
This object is achieved by a process for the production of polycarbonate by the phase boundary method, wherein a solution which contains polycarbonate is washed with an aqueous washing liquid, the washing liquid is separated from the solution which contains polycarbonate and the solvent is evaporated, wherein the organic solution which contains polycarbonate and the washing liquid are separated from each other after the first washing stage in a coalescer comprising a hydrophilic packing.
The present invention further relates to a process for the purification of solutions which contain polycarbonate, wherein the solution which contains polycarbonate is washed with an aqueous washing liquid, the washing liquid is separated from the solution which contains polycarbonate and the solvent is evaporated, wherein the organic solution which contains the polymer is separated from the aqueous washing liquid after the first washing stage in a coalescer comprising a hydrophilic packing.